1. Field of the Invention
This invention relates to length measuring devices and more particularly, to length measuring devices in which standard graduations of a planar magnetic scale formed in a magnetic grating are read by a detecting head.
2. Description of the Prior Art
Length measuring systems using a magnetic scale formed on a magnetic grating are generally based upon an incremental counting system, in which pulse signals for each unit displacement of a detecting head relative to a magnetic scale are counted to derive a mesurement value. A length measuring system based upon such an incremental counting system is simple in construction and relatively inexpensive to manufacture. However, previously calculated positional information is entirely lost in such systems with a loss of power. In addition, iron and other particles are likely to accumulate on the surface of the magnetic scale. If these particles are caught between the magnetic scale and the detecting head when it moves, they can damage the detecting head and the magnetic scale.